1. Field of the Invention
The present invention relates to magnetic particles used in a member for charging an object, a charging device using this charging member, a process cartridge and an electrophotographic apparatus, and they are applicable to devices such as copying machines, printers and facsimile machines.
2. Related Background Art
Heretofore, there are known many electrophotographic methods. In general, each of these methods employs a photoconductive material, forms an electrical latent image on a photosensitive member by any of various means, and then develops the latent image with a toner to form a visible image. If necessary, after transferring the toner image to a transfer material such as a paper, the toner image is fixed on the transfer material by heat or pressure to obtain a copy. Then, the toner particles remaining on the photosensitive member that are not transferred to the transfer material are removed from the photosensitive member by a cleaning process.
As a photosensitive member charging means by such an electrophotographic method, there is a charging method employing corona discharge, the so-called corotron or scotron. In addition, a charging method has been developed in which a charging member such as a roller, a fur brush or a blade is placed in contact with the surface of the photosensitive member, whereby discharge is formed in a narrow space in the vicinity of this contact to suppress the generation of ozone as much as possible, and this charging method is in practical use.
However, in the charging method utilizing the corona discharge, a great amount of ozone is generated particularly during the formation of the negative or the positive corona, and hence, it is necessary that a filter should be disposed on the electrophotographic apparatus to capture ozone, and this inconveniently increases the size and the running cost of the apparatus. Furthermore, in a method in which the charging is performed by placing a charging member such as a blade or a roller in contact with the photosensitive member, a problem that the toner melt-adheres to the photosensitive member tends to easily arise.
Therefore, a method in which the charging member is placed not in direct contact with but in the vicinity of the photosensitive member is being investigated. Examples of a member for charging the photosensitive member include the above-mentioned roller and blade, a brush and a long thin electroconductive plate having a resistance layer.
However, this method has a problem that it is difficult to control a distance between the charging member and the photosensitive member, which disturbs its practical use.
Thus, there has been investigated a technique which uses, as a charging member, the so-called magnetic brush formed by holding, with a magnet, magnetic particles having a relatively small load due to contact with the photosensitive member. Two charging methods using the magnetic particles in combination with the photosensitive member have been proposed. One is a method for charging the photosensitive member by forming a charge injection layer as a surface layer of the photosensitive member and then injecting an electric charge directly through contact with the charge injection layer. The other method employs discharge in the microscopic gaps between the surface of the photosensitive member and the magnetic particles using the usual photosensitive member.
In Japanese Patent Application Laid-Open No. 59-133569, a method is disclosed in which, for the magnetic particles used as the charging member, particles coated with iron powder are held on a magnet roll and charged by applying a voltage. However, with this method it is difficult to obtain a stable charging performance during continuous use. Japanese Patent Application Laid-Open No. 6-301265 proposes a construction that aims to stabilize resistance by replenishing the toner in order to standardize the amount of toner within the magnetic brush. These methods utilize discharge in the microscopic gaps, and problems such as damage to or degradation of the surface of the photosensitive member due to products from the discharge, and image slip or flow, which results easily at high temperature and high moisture levels, still remain.
Mixtures of relatively small diameter, highly electroconductive particles with relatively high resistance and low electroconductivity particles have also been proposed. Japanese Patent Application Laid-Open No. 6-258918 describes the use of a mixture of particles with volume resistance values of 10.sup.8 to 10.sup.10 .OMEGA.cm and diameters of 30 to 100 .mu.m with particles with volume resistance values under 10.sup.8 .OMEGA.cm and diameters of 30 to 100 .mu.m as particles for charging. Japanese Patent Application Laid-Open No. 6-274005 describes the use of a mixture of particles with volume resistance values of over 5.times.10.sup.5 .OMEGA.cm with particles with volume resistance values under 5.times.10.sup.4 .OMEGA.cm as particles for charging.
These offer good charging performance due to the diameter and resistance of the mixed particles, but when the resistance values of the particles largely differ, even if the diameters of the mixed particles are relatively close, during use the particles with low resistance will gather on the surface of the photosensitive member. As a result, even if initially the anti-pinhole quality was good, during use pinhole leaks tend to arise. If the particle diameters differ, the tendency for the low resistance particles to separate can be suppressed, but there is a strong tendency for particles with low resistance to leak out, particularly in low moisture environments.
Japanese Patent Application Laid-Open No. 8-6355 proposes a mixture of magnetic particles with bumpy surfaces and magnetic particles with smooth surfaces. It states that this will increase durability, but further increased durability is desirable.
Above, various proposals are mentioned, but as far as the present inventors understand the meaning of practical use, there are no examples of a magnetic brush being used as a charging member for photosensitive members in an electrophotographic apparatus such as a copying machine on the market. As for using magnetic particles as charging members for a photosensitive object, there has been insufficient examination into what materials are preferable and their effects, and development of the suitable structure for magnetic particles used for charging is desirable.
Conventionally, blade cleaning, fur brush cleaning, and roller cleaning have been used as cleaning processes in electrophotography. In all of these methods, the remaining transfer toner was mechanically swept out or dammed up and gathered into a waste toner container. Accordingly, problems resulting from such cleaning material being pushed across the surface of the photosensitive member arose.
For example, the photosensitive member could be scraped when the cleaning material is pushed against it with force, shortening the life of the photosensitive member. Also, the device must necessarily be made larger in order to equip it with such a cleaning device, an obstruction to the object of making the device more compact. From an ecological standpoint, a system in which waste toner does not result and the toner is efficiently used is desirable.
There is a technology called simultaneous development and cleaning, or development simultaneous with cleaning, or cleanerless, in which the development means is an actual cleaning means, in other words a system that performs cleaning through a development means but does not have a cleaning means for recycling and storing toner remaining on the photosensitive member after transfer, between the transfer device and the charging device and between the charging device and the developing device. For example, as described in Japanese Patent Application Laid-Open Nos. 59-133573, 62-203182, 63-133179, 64-20587, 2-51168, 2-302772, 5-2287, 5-2289, 5-53482, and 5-61383. However, these published technologies use a corona, a fur brush, or a roller as charging means, and are not satisfactory in all areas, such as contamination of the surface of the photosensitive member by products from discharge and nonuniformity of charge.
Thus, a cleanerless technology using a magnetic brush as charging member is being examined. For example, in Japanese Patent Application Laid-Open No. 4-21873 an image formation apparatus is proposed wherein a cleaning device is unnecessary because a magnetic brush to which an alternating voltage has been applied having a peak value exceeding the discharge limit value is used. Further, in Japanese Patent Application Laid-Open No. 6-118855, an image formation apparatus is proposed in which a magnetic brush charging cleaning device without an independent cleaning device is built on.
Metals such as iron, chromium, nickel, and cobalt, alloys or compounds of these, triiron tetroxide, .gamma.-ferric oxide, chromium dioxide, manganese oxide, ferrite, or manganese-copper alloys, or these materials coated with styrene resin, vinyl resin, ethylene resin, rosin modified resin, acrylic resin, polyamide resin, epoxy resin, or polyester resin, or a resin containing dispersed magnetic material microparticles are given as examples of the magnetic particles used.
However, the desirable form for the charging magnetic particles is not disclosed, and points such as the suitable magnetic particles for cleanerless method are left for further examination.